Apart from the usual Firefox security updates it contains some notable improvements compared to the 7.0 series. Here are the highlights:

We redesigned parts of the Tor Browser user interface. One of the major improvements for our users is our new Tor Launcher experience. This work is based on the findings published at 'A Usability Evaluation of Tor Launcher', a paper done by Linda Lee et al. At our work we iterated on the redesign proposed by the research, improving it even further. Here are the main changes we would like to highlight:

Welcome Screen

Our old screen had way too much information for the users, leading many of them to spend great time confused about what to do. Some users at the paper experiment spent up to 40min confused about what they needed to be doing here. Besides simplifying the screen and the message, to make it easier for the user to know if they need to configure anything or not, we also did a 'brand refresh' bringing our logo to the launcher.

Censorship circumvention configuration

This is one of the most important steps for a user who is trying to connect to Tor while their network is censoring Tor. We also worked really hard to make sure the UI text would make it easy for the user to understand what a bridge is for and how to configure to use one. Another update was a little tip we added at the drop-down menu (as you can see below) for which bridge to use in countries that have very sophisticated censorship methods.

Proxy help information

The proxy settings at our Tor Launcher configuration wizard is an important feature for users who are under a network that demands such configuration. But it can also lead to a lot of confusion if the user has no idea what a proxy is. Since it is a very important feature for users, we decided to keep it in the main configuration screen and introduced a help prompt with an explanation of when someone would need such configuration.

As part of our work with the UX team, we will also be coordinating user testing of this new UI to continue iterating and make sure we are always improving our users' experience. We are also planning a series of improvements not only for the Tor Launcher flow but for the whole browser experience (once you are connected to Tor) including a new user onboarding flow. And last but not least we are streamlining both our mobile and desktop experience: Tor Browser 7.5 adapted the security slider design we did for mobile bringing the improved user experience to the desktop as well.

On the security side we enabled content sandboxing on Windows and fixed remaining issues on Linux that prevented printing to file from working properly. Additionally, we improved the compiler hardening on macOS and fixed holes in the W^X mitigation on Windows.

We finally moved away from Gitian/tor-browser-bundle as the base of our reproducible builds environment. Over the past weeks and months rbm/tor-browser-build got developed making it much easier to reproduce Tor Browser builds and to add reproducible builds for new platforms and architectures. This will allow us to ship 64bit bundles for Windows (currently in the alpha series available) and bundles for Android at the same day as the release for the current platforms/architectures is getting out.

I hope the new addon GUI in next FF59 release midjear? is somewhat ...usable.

When you look at https://hackademix.net
blog from author of NoScript, the new GUI api mozilla is forcing. You can't escape and it forces a lot of confusion.
The classic menu is practical, the new one ...TEEMING only? Scary)-:

Doesn't work on Windows Vista x64.
Fully broken. Nothing works, Tor doesn't launch, Mozilla doesn't respond. Plenty errors.
I have only Vista Windows on all my PCs and some on XP (didn't test on XP though). And I'm not going to update to later Windows with MS spying on you. Never ever.
Mind that Vista works great on new AMD Ryzen, so that it ! Also recommended plenty people and companies to stay on Vista... best Windows ever.
I may have to download a 7.0 version of Torbrowser and stay on it, deactivate updates.
Mind that Mozilla bigger than 52 wont work on Vista too.

What does "fully broken" mean and "Tor doesn't launch"? Do you get error messages during start? Does this happen with a clean, new Tor Browser as well? How far in the start-up process are you coming with that one? Are you seeing some dialogs/windows and the browser is crashing then?

Do you have some antivirus/firewall tool that could block Tor Browser from running?

Vista is so secure that it's not needed. Ever asked yourself why MS replaced it ???
Just have to let it as is with it's security features. People were annoyed by the fact that they had security alerts and couldn't do anything they wanted, especially in user mode.
Windows 7 is Vista with security features dropped and lot of pathes to help.
Windows 10 is Windows 7 + ugly and stupid interface + spyware + all security features on (besides MS spying on you all time) and forced pathes which put their security features and spyware back every time you touch them.
I have Vista systems running everyday without any reinstall, no Virus, since when Vista came out 10 years ago. Most are C2Q with 8GB RAM, updated graphics (Nvidia Maxwell) so they run pretty well.

And I agree with the guy who says Firefox is becoming junkware like Chrome and all the stuff running after commercial features.
Fact is this has become so since Brendan Eich has been fired for private misbehaviour, that kind of stresses who are the people in charge now at Mozilla. Branching off at 52.x level is the good move for Torbrowser.
I hope TorBrowser is rapidly fixed to work on Vista again.

Vista doesn't even deny mapping pages to null addresses... I would call that a nasty security issue. Not to mention all the "security" features Vista added (read: UAC) are completely broken. UAC is a joke. 7 is slightly better, 8 is a lot better, 10 is better still (it doesn't render fonts in kernelspace anymore, though it still renders some windows and scrollbars that way...). 10 also has a boatload of new security features that make EMET practically obsolete. Now, is Windows 10 "good"? No. It's a privacy nightmare. But to say that Vista is secure in any way is just insane. Better than XP, but still shit.

Browser launches but Tor doesn't seem to. Very weird. But even browser is half down. It must be killed to close the opened Windows. There is no response when trying to surf. It should be something even if Tor is down (since it should try to connect the proxy). It's simply as if the people compiling never tested on Vista (x64). Version 7.0.11 works great.

7.5 "Doesn't work on Windows Vista x64"--the same for me. A connection is made, but when a window opens no page loads. I tried a new identity and received this message: "TypeError: frameLoader.tabParent is null". I restored Tor to its original settings (no add-ons etc.) and still no page loads. I uninstalled Tor 7.5 and reinstalled it and was successful up to pressing "Finish" when a blank window opens and no page loads.

Okay. Usually I click on "New Tor circuit for this site" and, on the new circuit, the site comes in. Often I have to click "new tor circuit" a couple of times, but then I guess the right countries are circuited and it comes up, no problem. But...
yesterday and today no matter how many times I do that I can't get the site to come up. A funny thing, I started to notice, was that Bulgaria kept being the first IP address, after "This browser", then I saw that it was the exact same IP address, in Bulgaria, every time. Something must be wrong, if every time I ask Tor to reroute me through a new set of onion-layers I keep being sent through the same one address.

The clearnet sites you visit can see only the exit node, the final IP, not the other nodes of your circuit to it. Many sites choose to block all known Tor exit nodes reasoning that Tor users who might behave maliciously cannot be blocked individually. Some help sites for network security admins compile lists of all Tor exit node IPs that website admins can import into their firewalls to block Tor traffic. Craigslist is doing something like this. Some exit nodes might not be blocked because they haven't been discovered yet by the people who compile the lists. The IPs of all Tor nodes except bridge nodes are published by the Tor network so that circuits can be created in the first place.

Some clearnet sites choose not to block but put Captchas, some requiring Tor users to enable JavaScript, as gates in the path of all Tor users before allowing them to view the site. Cloudflare's services are notorious for blocking and monitoring Tor users. Tor Project has FAQ pages describing options for website administrators who are considering to block or restrict Tor traffic:https://www.torproject.org/docs/faq-abuse.html.en

I keep noticing that certain otherwise valuable activist and independent news media sites remain vulnerable to at least one of these.

There is a small conundrum here: is it better to insist on safer crypto at the expense of confusing puzzled non-power users who don't understand why they get a message warning that some site's encryption is broken (and probably fear that the site has suffered an intrusion, rather than that it is not well maintained from a security perspective).

At the very least, I hope TP leadership will try to ensure that this kind of default configuration decision is regularly revisited to take account of increasing cybersecurity mindfulnees among activists and news organizations, and current threats as observed in the wild (or reasonably expected to become common in the near future, e.g. Meltdown attacks on unpatched Cloud servers with Intel processors).

Well, well, gk. As you're saying there's no censorship in the blog, where is my reply here, disproving your comment? It seems this blog 'eats' some comments, because other comments, posted that day, are visible now.

This is not one attack but a fundamental flaw which will enable a whole class of attacks. Currently known proof-of-concept Meltdown attacks require, AFAIK, that the attacker use some "ordinary exploit" to gain a local presence on the attacked device--- most likely a Cloud Computing server hosting sensitive information about millions of people, but quite possibly a workstation or laptop used by a reporter, activist, doctor, lawyer etc. So it is not just USG agencies (which have moved aggressively into the cloud) which should be very worried; at-risk individuals should also try hard to take proactive countermeasures.

In the case of attacks on at-risk individuals, some experts have specifically stated that exploits of zeroday flaws or unpatched flaws in javascript are one of the most likely ways that attackers will seek to gain a local presence on a PC in order to begin a Meltdown attack seeking to capture the most dangerous information, such as cryptographic material residing in the kernel space memory of a running PC with a mounted encrypted external drive holding (for example) casework for a sensitive lawsuit.

IMO the prospect of Meltdown attacks does at the very least demand a discussion of whether an "emergency adjustment" of the default slider configuration might be warranted.

After installing update, Avast Antivirus saying that 7.5 update is malware.
I clicked that it is Not Harmful in some kind of popup in avast.
Is my 7.5 update safe??
Because I updated from that Green Tor button Check Update.

I guess what's happening is that your Avast tool has never seen this Tor Browser before and now thinks "Holy shit this must be malware then". Our updates are signed so that someones tampered with them and your browser is still working is very unlikely.

"I updated from that Green Tor button Check Update."
You are very probably OK. Tor Browser downloads updates from torproject.org by HTTPS over Tor. The variable is app.update.url in about:config. Only bad if Tor server or website certificate is hacked or exe changed after reproducible build and before signing. Not probable.

The new Tor Launcher UI is obviously an improvement, but once the user configures it (in most use cases once and forever) the UI wastes a lot of screen space to merely display a logo, a progress bar, and progress info.
I admit the uniformity of window size between configuration stage and connecting stage is fancy (the uniformity which in most cases the user will notice only once), but not enough to redeem the waste of screen space.